Many conventional refrigeration systems used in refrigerator appliances, for example, rely on a sealed configuration allowing refrigerant flow through a circuit with a compressor, a condenser, a pressure reduction device and an evaporator. When the system is called on to cool a refrigeration compartment in the appliance, the compressor operates to increase the pressure and temperature of the refrigerant existing in a vapor state. The refrigerant vapor then travels through the condenser, where it is condensed into a liquid state at constant pressure and temperature. The liquid refrigerant then passes through the pressure reduction device, and experiences a significant drop in pressure. This results in evaporation of the refrigerant and a significant decrease in the temperature of the refrigerant. The refrigerant, now in a liquid/vapor state, passes through the evaporator. There, the refrigerant is typically fully vaporized by warmer air that is passed over the evaporator from the compartment intended to be cooled. The process then repeats as the refrigerant vapor is suctioned back into the compressor.
In general, conventional refrigeration systems operate at a high efficiency when the refrigerant exiting the condenser is in a completely liquid state and the refrigerant exiting the evaporator is in a completely vapor state. These refrigerant conditions are possible during steady-state operation of the compressor during a cycle of cooling one or more refrigeration compartments in the appliance. Compressors used in conventional refrigeration systems are also designed and sized to operate under a variety of ambient temperature and humidity conditions (e.g., tropical environments), and to properly cool refrigeration compartments in the appliance under a variety of transient conditions (e.g., a large mass of hot food has been introduced into the appliance).
Consequently, conventional systems rarely operate in a continuous, steady-state mode with high efficiency. At certain times, the system turns the compressor OFF when cooling of a compartment is not necessary. The system might later turn the compressor back ON when cooling is necessary because, for example, the temperature in a refrigeration compartment has exceeded a set point. During these down periods, however, refrigerant will re-distribute in the circuit. Often refrigerant in a liquid state will migrate through the circuit and pool in the evaporator. Consequently, the system will need some period of time to re-distribute the refrigerant within the circuit upon start-up of the compressor when cooling of a compartment is required. During these periods, the system is operating far below the efficiencies achieved when the refrigerant is in a completely liquid state at the exit of the condenser and completely vapor state at the exit of the evaporator.
Efficiency losses on the order of 5-10% may result from the effects of refrigerant migration during compressor OFF cycles in conventional refrigeration systems. The refrigerant is often not in an ideal state throughout the refrigerant circuit during the initial phase of a compressor ON cycle. Moreover, when warm refrigerant has migrated from the condenser to the evaporator during a period when the compressor is not operating, efficiency is lost from heat transfer of the warmer refrigerant in the evaporator to the refrigeration compartment. The use of heat exchanging members (e.g., suction line heat exchangers and intercoolers) in some refrigeration systems also can exacerbate the problem. Heat exchangers in contact with the compressor inlet and evaporator inlet lines can improve system efficiency during steady-state operation. However, they tend to prolong the effects of refrigerant migration during compressor OFF cycles by inhibiting the mass flow rate of the refrigerant through the refrigerant circuit upon the initiation of a compressor ON cycle.
Consequently, what is needed is a system that not only maximizes steady-state efficiency, but also has improved efficiency during the initial phase of a compressor ON cycle. Conventional systems are not designed to address refrigerant migration. Indeed, many conventional systems exacerbate the problem by employing heat exchanging elements designed to only improve efficiency during steady-state operation of the compressor.
The refrigerator appliances, and methods associated with operating them, related to this invention address these problems. They allow for the design of control logic that considers the location and condition of the refrigerant in the refrigerant circuit. When refrigerant has disadvantageously migrated within the circuit during a compressor OFF-cycle, for example, the appliances and methods according to the invention can operate to improve overall system efficiency. They achieve these gains by taking an unconventional approach to the operation of the appliance during the relatively short, initial phase of a compressor-ON cycle. Very generally, these appliances and associated methods are structured to allow for operation of the appliance at a sub-optimal thermodynamic efficiency during the beginning of a compressor-ON cycle. The immediate emphasis is on an efficient and speedy re-distribution of the refrigerant. Accordingly, the appliance can move into a more efficient, steady-state operational regime at an earlier time than conventional systems, thereby improving overall system efficiency.